Method for preventing re-emission of mercury from a flue gas desulfurization system

ABSTRACT

An improvement in the method for preventing re-emissions of mercury from a wet flue gas desulfurization (FGD) system by addition of an additive to the FGD scrubber liquor which interacts in the system scrubber with mercury present in the flue gas to curtail the mercury re-emissions; the mercury re-emissions are reduced to substantially zero by use of an additive selected from one or more members of the group consisting of a dithiol, a dithiolane, and a thiol having a single thiol group and either an oxygen or a hydroxyl group.

PRIORITY STATEMENT UNDER 35 U.S.C. §119 & 37 C.F.R. §1.78

This non-provisional application claims priority based upon prior U.S.Provisional Patent Application Ser. No. 61/279,550 filed Oct. 22, 2009in the name of William A. Steen, John E. Currie, Gary M. Blythe,Jennifer L. Paradis and David W. DeBerry entitled “Method for Preventingthe Re-emission of Elemental Mercury from a Flue Gas DesulfurizationSystem,” the disclosure of which is incorporated herein in its entiretyby reference.

BACKGROUND OF THE INVENTION

This invention relates generally to reducing the emission of vapor phasemercury in flue gas emissions, thereby restoring air quality andenhancing the environment, and more specifically relates to minimizingthe re-emission of mercury from a wet flue gas desulfurization (“FGD”)system through the addition of additives during desulfurization.

Several industrial processes, including the conversion of coal to power,include scrubbers for removal of acid gases, such as hydrochloric acidand sulfur dioxide. Hydrochloric acid is typically removed bydissolution in water and the resulting liquor is then neutralized with asubstance such as lime. Sulfur dioxide is typically removed with a wetflue gas desulfurization scrubber, wherein the flue gas containingsulfur dioxide is placed in contact with water containing an alkalinematerial, such as limestone, lime, magnesium compounds or sodiumcompounds. In the scrubber, the alkaline material reacts with the sulfurdioxide to form a neutral compound such as calcium sulfate dehydrate(i.e. gypsum).

In some feed stocks, such as coal, mercury is present in smallquantities. The mercury is oxidized to varying degrees when the coal iscombusted, or it can be oxidized in separate unit operations designedfor that purpose or for the removal of other pollutants. The portion ofthe mercury that remains as elemental mercury)(Hg⁰) is gaseous and iswater insoluble and will, therefore, pass through the scrubber withoutbeing affected. There are a number of methods known in the art forremoving elemental mercury from flue gas.

The portion of the mercury that is oxidized is water soluble (e.g. ionicmercury) and will be absorbed into the scrubber solution. The oxidizedmercury will leave the scrubber with the scrubber solution by an outletliquid or solid stream. If the mercury stays in its oxidized,water-soluble form it can be removed by the flue gas desulfurizationwater slurry, commonly referred to as the “FGD liquor”, is dischargedinto the environment. Unfortunately, the oxidized mercury is sometimesreduced back to its elemental form, vaporized, and released into theatmosphere as part of the scrubbed flue gas in a process referred to as“mercury re-emissions.”

Several strategies are currently being developed to control there-emission of mercury, many involving the addition of proprietarychemicals to the FGD liquor intended to either keep the mercury in itssoluble form or precipitate it as a solid. Other control strategiesinvolve the use of materials to adsorb mercury (e.g. activated carbon orgold).

SUMMARY OF THE INVENTION

The present invention provides a simple, efficient and cost effectivemethod for minimizing or eliminating the amount of oxidized mercurypresent in the wet flue gas desulfurization system that is reduced andsubsequently re-emitted. More specifically, by adding certain chemicaladditives to FGD liquors mercury re-emission is virtually eliminatedduring desulfurization. In one embodiment of the present invention, theadditive can be introduced into the scrubber with the limestone slurrywhich is added to control SO₂.

Additives useful in the present invention are selected from one or moremembers of the group consisting of certain thiols, a dithiol or adithiolane; many additives also have oxygen-containing functionalgroups. For example, the dithiol may include a doubly-bonded oxygen (O═)and/or a hydroxyl group (—OH). Preferable dithiols include 1,8octanedithiol, dimercaptosuccinic acid, and 2,3-dimercaptopropanol. Thedithiolane may include a doubly-bonded oxygen (O═) and a hydroxyl group(OH—); and preferable thiolanes include 2-methyl 1,3-dithiolane and1,2-dithiolane-3-valeric acid. Preferable thiols are those having asingle thiol group, and either an oxygen or a hydroxyl group. Theseinclude mercaptoacetic acid and the sodium salt of mercaptoacetic acid.

The foregoing has outlined rather broadly certain aspects of the presentinvention in order that the detailed description of the invention thatfollows may be better understood. Additional features and advantages ofthe invention will be described hereinafter which form the subject ofthe claims of the invention. It should be appreciated by those skilledin the art that the conception and specific embodiment disclosed may bereadily utilized as a basis for modifying or designing other structuresor processes for carrying out the same purposes of the presentinvention. It should also be realized by those skilled in the art thatsuch equivalent constructions do not depart from the spirit and scope ofthe invention as set forth in the appended claims.

DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic block diagram depicting a typical wet flue gasdesulfurization system, wherein mercury re-emission additives areintroduced to the wet flue gas desulfurization process with the reagentstream added for SO₂ removal;

FIG. 2 is a schematic block diagram depicting a bench-scale wet flue gasdesulfurization system;

FIG. 3 is a graph showing the effect of a variety of additives on Hg⁰concentration in the outlet flue gas stream as a function of elapsedtime;

FIG. 4 is a graph showing the effect of mercaptoacetic acid on Hg⁰concentration in the outlet flue gas stream as a function of elapsedtime; and

FIG. 5 is a graph showing the distribution of mercury in the solid,liquid and gas phase as a function of elapsed time during the activeaddition of mercaptoacetic acid.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to improved methods for minimizing there-emission of mercury in a flue gas desulfurization system. Theconfiguration and use of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of contexts other than traditional fluegas desulfurization processes. Accordingly, the specific embodimentsdiscussed are merely illustrative of specific ways to make and use theinvention, and do not limit the scope of the invention. In addition, thefollowing terms shall have the associated meaning when used herein:

“power plant” means any plant using furnaces, boilers or heatersconsuming coal, oil or any solid, liquid or gaseous fuel from which fluegas is discharged;

“reagent solids” means any reagent known in the art for use in a FGDprocess, including hydrated lime, limestone, soda ash, nahcolite, anddolomite; and

“wet scrubbing system” and similar terms mean any system designed forsolid particle or SO_(x) removal during flue gas discharge.

FIG. 1 is a schematic block diagram depicting a typical wet flue gasdesulfurization system 10, wherein one or more mercury re-emissionadditives 12 are introduced to the wet flue gas desulfurization processwith the reagent stream added for SO₂ removal. The reagent liquor isprepared as a slurry at station 14 from water and reagent solids 18, andis provided to absorber 20. In this embodiment, the mercury re-emissionadditives of the invention are introduced to the wet flue gasdesulfurization process with the slurried reagent stream 22 added forSO₂ removal. The flue gas 24 to be treated is provided to the absorber20, as is makeup water 26, where contact between the inlet flue gas andthe slurry results in the scrubbing of the flue gas. The treated fluegas 28 is discharged from the absorber 20 as a purified product.

The spent slurry is bled off at 30 and dewatered at 32, with theresulting solid by-product 34 being disposed of or available as asaleable commodity. The overflow and filtrate 36 from the dewatering isreturned to the reagent preparation station 14 for use in slurrypreparation, and returned to the absorber 20 as additional makeup water.With the exception of the re-emission additives 12, all of the foregoingprocedures and apparatus are known in the prior art.

The method of the present invention represents a low cost means ofadding mercury re-emission additives to the process at a rate thatalready varies based on the amount of SO₂ being absorbed. Since undercontrolled conditions the amount of SO₂ absorbed is an indicator of theamount of coal being combusted, the rate at which reagent is added tothe process will correspond to the rate of mercury absorption.

FIG. 2 is a schematic block diagram depicting a laboratory scale wetflue gas desulfurization system 40. The system includes a reactor vessel42 and absorber tower 44 containing FGD flue gas 56 and liquor 46.Feedback control loops are used: to maintain a constant pH through theaddition of sodium hydroxide (NaOH) from feed tank 48; to maintain thesulfite concentration through the addition of hydrogen peroxide (H₂O₂)from feed tank 50; and otherwise to maintain a constant temperature. Theliquor 46 from the reaction tank is passed via a loop 52 includingrecirculation pump 54, to the tower 44 where it forms a liquid column.The flue gas 56 provided to the tower 44 via inlet 58 exits at thebottom 60 of the column, and after bubbling upwardly through the liquor,where it is scrubbed by the liquor, exits at outlet 62 as the purifiedwaste gas 64.

Tests were conducted using the system described above and shown in FIG.2 to compare the efficacy of the present invention in comparison withother additives. The inlet and outlet mercury concentrations weremeasured in the flue gas at analyzers 66 and 68. Both total mercury(oxidized and elemental) and elemental mercury (Hg⁰) were measured in asemi-continuous manner. All measurements were performed with thereaction tank operating at 131° F. and pH=5. Other parameters associatedwith the liquor or flue gas are shown below:

Liquor

-   -   100 mM C1⁻,    -   50 mM SO₄ ⁻²,    -   15 mM Ca⁺²,

Flue Gas Inlet

-   -   15-25 μg/Nm³ HgCI₂, with trace amounts of Hg⁰    -   12% CO₂,    -   3% O₂,    -   1000 ppm SO₂,    -   15 ppm HC1,    -   Balance nitrogen, and    -   24 L/min nominal total flow (dry basis).    -   Note that a solid phase was not present in these laboratory        scale tests, unlike wet FGD units servicing utilities. In each        test except the baseline test, a Hg re-emission additive was        included in the liquor as a single spike at 0.05 mM. All        chemicals were added to the liquid phase and the simulated flue        gas was bubbled through the absorber tower until steady state        was reached.

The results of four different experiments are shown in FIG. 3, which isa plot of elemental mercury (Hg⁰) in the outlet flue gas stream as afunction of elapsed time. From time 0 until approximately 3 hours, theHg⁰ concentration was at or below detection limits (˜1 μg/Nm³). Atapproximately 3 hours, the amount of mercury being introduced into thesystem via the inlet flue gas was supplemented with an injection ofsoluble Hg⁺² in the form of Hg(C1O₄)₂. Mercury re-emissions are seen toimmediately increase after this mercury spike.

In the baseline case (curve A), re-emissions are steady at nominallytheir initial values (˜10 μg/Nm³) for the duration of the experiment.Contrast the baseline case with the lipoic acid(1,2-dithiolane-3-valeric acid) additive, the results for which aredepicted in curve D; or with curve B which depicts the results where theadditive is 2,3-dimercaptopropanol. In these latter two instances theelemental mercury concentration initially increases with this surge inoxidized mercury but the re-emissions are quickly eliminated, asevidenced by the outlet elemental mercury concentration falling to thedetection limit (˜1 μg/Nm³). Among the additional additives found to beeffective in the present invention are the compounds dimercaptosuccinicacid; 1,8-octanedithiol; 2-methyl 1,3-dithiolane; mercaptoacetic acid,and the sodium salt of mercaptoacetic acid.

Two other additives (4-aminothiophenol and 2-mercapto benzimidazole)that were tested are also shown in FIG. 3 (curves C and E). These twochemicals were not seen to affect emissions as effectively as lipoicacid and 2,3-dimercaptopropanol; in fact, the mercury re-emission trendswere the same as the baseline case for these additives. Both of theseadditives are technically organothiols, and yet both are ineffective forpreventing mercury re-emissions. Thus, it will be clear that the groupof additives delineated in the present invention possess the unexpectedand unique properties that are effective for such a result. There arediscernable chemical distinctions between the two groups of additivesshown in FIG. 3: The chemicals used in curves B and D that eliminatedre-emissions contain a dithiol or dithiolane group as well as adoubly-bounded oxygen (═O) and/or a hydroxyl (—OH) group, whereas theineffective chemicals 4-aminothiophenol and 2-mercapto benzimidazoleused in curves C and E contain a single thiol along with an aminofunctional group.

An additional additive, mercaptoacetic acid (MAA), was tested. In thiscase, the experiments were performed using a slurry from an operatingutility. Both the liquid and solid phases were used in an effort to moreclosely mimic conditions encountered in full scale wet FGD system. Theexact composition of the solid and liquid was not determined. Some ofthe test conditions were slightly modified to match the conditions of autility scrubber; the changes are noted below.

Liquor

-   -   pH 5.5

Flue Gas

-   -   1200 ppm SO₂    -   5 ppm HCl    -   265 ppm NOx

In addition, instead of using sodium hydroxide and hydrogen peroxide tocontrol pH and sulfite levels, reagent grade calcium carbonate andsparged air were used, respectively. All other conditions were the sameas the tests presented in FIG. 3.

FIG. 4 shows the gas phase outlet elemental mercury concentration as afunction of the elapsed time since the start of the test. Since onlytrace amounts of elemental mercury are introduced in the gas phase, thehigh concentration of mercury at the scrubber outlet is clearly theresult of re-emissions. Mercaptoacetic acid was spiked into the systemto quickly introduce an inventory in the reactor and continued at aconstant feed. The mercaptoacetic acid spike occurs at approximately 2hours and is annotated on the graph. The mass of mercaptoacetic acidintroduced corresponds to a concentration of 27 ppm or 0.3 mM. The testlasted approximately 6 hours after MAA was initially being added to thesystem.

The addition of mercaptoacetic acid to the system significantly reducedmercury re-emissions. As previously discussed, any gas phase mercuryconcentration value below ˜1 μg/Nm³ can be considered experimental noiseand below the detection limit of the instrument. The final concentrationof mercaptoacetic acid added during the test resulted in a concentrationto 400 ppm or 4.3 mM by the end of the test, assuming the additiveremained in the liquid phase and did not degrade.

Of interest is the fact that, as mercaptoacetic acid was being activelyadded to the FGD system, the mercury partitioning was affected, as shownin FIG. 5. The distribution of mercury in the solid, liquid and gasphase is shown in FIG. 5 as a function of elapsed time. At the beginningof the test, all of the mercury is in the solid phase (400 μg of mercurycorresponds to a concentration of 1 μg/g). After the mercaptoacetic acidis introduced, the mercury is seen to partition to the liquid phase. Thehighest liquid phase mercury concentration seen at 2.5 hours is 50 μg/L.Relative to the mercury in the liquid and solid phases, little mercuryexits the system as re-emitted mercury, which is evidenced in FIG. 4 anddemonstrates the effectiveness of MAA.

A small parametric test matrix was also performed using mercaptoaceticacid. The pH and chloride levels were varied. The pH was tested at 4.7,5.2, and 5.7, and chloride was tested at 5 mM and 50 mM levels. Insteadof performing the tests with samples from a utility wet FGD, a syntheticslurry was prepared using calcium sulfate as the solid material. Withthe exception of the chloride level and pH, the liquid phase chemistryof the slurry was the same as it was for the results presented in FIG.3. Reagent grade calcium carbonate and air were used to control pH andsulfite levels. The addition of mercaptoacetic acid, either as a singlespike or metered into the system over time, was effective at controllingre-emissions. The results of these experiments are summarized in Table1.

TABLE 1 Outlet Hg Lowest Achievable Percent Hg Chloride concentrationOutlet Hg Partition Concentration before Concentration After to Solid pH(mM) MAA (μg/Nm³) MAA (μg/Nm³) Phase 4.7 5 37 <1  97% 4.7 50 3 <1 100%5.2 5 34 <1  27% 5.2 50 5 1.5  5% 5.7 5 13 1  93% 5.7 50 4 <1  98%

For these conditions, an apparent ratio of 200-250 to 1 mercaptoaceticacid to mercury in the system seemed effective at eliminatingre-emissions. It was observed that more mercaptoacetic acid was requiredat higher pH, while the mercury partitioning favored the liquid phasefor the tests performed at pH 5.2.

While the present system and method has been disclosed according to thepreferred embodiment of the invention, those of ordinary skill in theart will understand that other embodiments have also been enabled. Eventhough the foregoing discussion has focused on particular embodiments,it is understood that other configurations are contemplated. Inparticular, even though the expressions “in one embodiment” or “inanother embodiment” are used herein, these phrases are meant togenerally reference embodiment possibilities and are not intended tolimit the invention to those particular embodiment configurations. Theseterms may reference the same or different embodiments, and unlessindicated otherwise, are combinable into aggregate embodiments. Theterms “a”, “an” and “the” mean “one or more” unless expressly specifiedotherwise. The term “connected” means “communicatively connected” unlessotherwise defined.

When a single embodiment is described herein, it will be readilyapparent that more than one embodiment may be used in place of a singleembodiment. Similarly, where more than one embodiment is describedherein, it will be readily apparent that a single embodiment may besubstituted for that one device.

In light of the wide variety of flue gas systems known in the art, thedetailed embodiments are intended to be illustrative only and should notbe taken as limiting the scope of the invention. Rather, what is claimedas the invention is all such modifications as may come within the spiritand scope of the following claims and equivalents thereto.

None of the description in this specification should be read as implyingthat any particular element, step or function is an essential elementwhich must be included in the claim scope. The scope of the patentedsubject matter is defined only by the allowed claims and theirequivalents. Unless explicitly recited, other aspects of the presentinvention as described in this specification do not limit the scope ofthe claims.

1. A method for reducing re-emissions of mercury from a wet flue gasdesulfurization system comprising: adding an additive to scrubber liquorin the flue gas desulfurization system which interacts with mercurypresent in the flue gas to curtail the mercury re-emissions, whereinsaid additive is a dithiol, and wherein the dithiol has either an oxygengroup or a hydroxyl group.
 2. A method for reducing re-emissions ofmercury from a wet flue gas desulfurization system comprising: adding anadditive to scrubber liquor in the flue gas desulfurization system whichinteracts with mercury present in the flue gas to curtail the mercuryre-emissions, wherein said additive is 2,3-dimercaptopropanol.
 3. Amethod for reducing re-emissions of mercury from a wet flue gasdesulfurization system comprising: adding an additive to scrubber liquorin the flue gas desulfurization system which interacts with mercurypresent in the flue gas to curtail the mercury re-emissions, whereinsaid additive is 1,8-octanedithiol.
 4. A method for reducingre-emissions of mercury from a wet flue gas desulfurization systemcomprising: adding an additive to scrubber liquor in the flue gasdesulfurization system which interacts with mercury present in the fluegas to curtail the mercury re-emissions, wherein said additive is adithiolane.
 5. The method of claim 4, wherein the dithiolane has eitheran oxygen group or a hydroxyl group.
 6. The method of claim 4, whereinthe dithiolane is 1,2-dithiolane-3-valeric acid.
 7. The method of claim4, wherein the dithiolane is 2-methyl 1,3-dithiolane.
 8. A method forreducing re-emissions of mercury from a wet flue gas desulfurizationsystem comprising: supplying an additive to scrubber liquor in the fluegas desulfurization system in a sufficient quantity to reduce there-emission of mercury; wherein said additive is a dithiol, wherein thedithiol has either an oxygen group or a hydroxyl group; and scrubbingthe wet flue gas with the scrubber liquor containing the additive.
 9. Amethod for reducing re-emissions of mercury from a wet flue gasdesulfurization system comprising: supplying an additive to scrubberliquor in the flue gas desulfurization system in a sufficient quantityto reduce the re-emission of mercury; wherein said additive is2,3-dimercaptopropanol; and scrubbing the wet flue gas with the scrubberliquor containing the additive.
 10. A method for reducing re-emissionsof mercury from a wet flue gas desulfurization system comprising:supplying an additive to scrubber liquor in the flue gas desulfurizationsystem in a sufficient quantity to reduce the re-emission of mercury;wherein said additive is 1,8-octanedithiol; and scrubbing the wet fluegas with the scrubber liquor containing the additive.
 11. A method forreducing re-emissions of mercury from a wet flue gas desulfurizationsystem comprising: supplying an additive to scrubber liquor in the fluegas desulfurization system in a sufficient quantity to reduce there-emission of mercury; wherein said additive is a dithiolane, whereinthe dithiolane has either an oxygen group or a hydroxyl group; andscrubbing the wet flue gas with the scrubber liquor containing theadditive.
 12. A method for reducing re-emissions of mercury from a wetflue gas desulfurization system comprising: supplying an additive toscrubber liquor in the flue gas desulfurization system in a sufficientquantity to reduce the re-emission of mercury; wherein said additive is1,2-dithiolane-3-valeric acid; and scrubbing the wet flue gas with thescrubber liquor containing the additive.
 13. A method for reducingre-emissions of mercury from a wet flue gas desulfurization systemcomprising: supplying an additive to scrubber liquor in the flue gasdesulfurization system in a sufficient quantity to reduce there-emission of mercury; wherein said additive is 2-methyl1,3-dithiolane; and scrubbing the wet flue gas with the scrubber liquorcontaining the additive.